In the past, the material which has been used for the automotive turbo housing etc. has been required to exhibit an extremely high-temperature strength under a high-temperature environment which reaches as much as 800° C., so stainless cast steel has been used. However, in the face of the demand for reducing costs in recent years, production of parts from steel sheet, which enables more inexpensive production than production of parts by machining cast steel, has been proposed, and development efforts thereof are underway. As stainless steel sheet which is used under high-temperature environments, austenitic stainless steel such as SUS310S is being used. However, in recent years, the requirements on the performance of the materials used such as the high-temperature strength and oxidation resistance have become severer and can no longer be satisfied by SUS310S.
The characteristics which are sought for materials relevant to turbocharger are high-temperature strength and creep characteristics. In the creep characteristics, a certain magnitude of deformation after a certain time is considered more important than lifetime. Further, working is essential, so a certain degree of workability is also demanded.
The invention which is disclosed in PLT 1 improves the creep strength by addition of P. However, addition of P has the problem of reducing the weldability and creep ductility. Further, there are also concerns over lowering the corrosion resistance. The invention which is disclosed in PLT 2 adds an REM, in particular Nd, in addition to P so as to improve the creep ductility and weldability. However, addition of an REM invites a rise in cost.
PLTs 3 and 4 disclose austenitic stainless steel which is excellent in heat resistance. Here, these disclose adjusting the component elements with each other to obtain steel which is excellent in heat resistance, in particular which is excellent in embrittling cracking resistance of the weld zone. However, the creep characteristics disclosed in PLTs 3 and 4 are evaluated only at 650° C. or less and are not evaluated at 800° C.